65 research outputs found
The cyanobacterial chlorophyll-binding-protein IsiA acts to increase the in vivo effective absorption cross-section of PSI under iron limitation
No full textIron availability limits primary production in >30% of the world’s oceans; hence phytoplankton have developed acclimation strategies. In particular, cyanobacteria express IsiA (iron-stress-induced) under iron stress, which can become the most abundant chl-binding protein in the cell. Within iron-limited oceanic regions with significant cyanobacterial biomass, IsiA may represent a significant fraction of the total chl. We spectroscopically measured the effective cross-section of the photosynthetic reaction center PSI (?PSI) in vivo and biochemically quantified the absolute abundance of PSI, PSII, and IsiA in the model cyanobacterium Synechocystis sp. PCC 6803. We demonstrate that accumulation of IsiA results in a ?60% increase in ?PSI, in agreement with the theoretical increase in cross-section based on the structure of the biochemically isolated IsiA-PSI supercomplex from cyanobacteria. Deriving a chl budget, we suggest that IsiA plays a primary role as a light-harvesting antenna for PSI. On progressive iron-stress in culture, IsiA continues to accumulate without a concomitant increase in ?PSI, suggesting that there may be a secondary role for IsiA. In natural populations, the potential physiological significance of the uncoupled pool of IsiA remains to be established. However, the functional role as a PSI antenna suggests that a large fraction of IsiA-bound chl is directly involved in photosynthetic electron transport
Interactions between Thermal Acclimation, Growth Rate, and Phylogeny Influence Prochlorococcus Elemental Stoichiometry.
No full textVariability in plankton elemental requirements can be important for global ocean biogeochemistry but we currently have a limited understanding of how ocean temperature influences the plankton C/N/P ratio. Multiple studies have put forward a 'translation-compensation' hypothesis to describe the positive relationship between temperature and plankton N/P or C/P as cells should have lower demand for P-rich ribosomes and associated depressed QP when growing at higher temperature. However, temperature affects many cellular processes beyond translation with unknown outcomes on cellular elemental composition. In addition, the impact of temperature on growth and elemental composition of phytoplankton is likely modulated by the life history and growth rate of the organism. To test the direct and indirect (via growth rate changes) effect of temperature, we here analyzed the elemental composition and ratios in six strains affiliated with the globally abundant marine Cyanobacteria Prochlorococcus. We found that temperature had a significant positive effect on the carbon and nitrogen cell quota, whereas no clear trend was observed for the phosphorus cell quota. The effect on N/P and C/P were marginally significantly positive across Prochlorococcus. The elemental composition and ratios of individual strains were also affected but we found complex interactions between the strain identity, temperature, and growth rate in controlling the individual elemental ratios in Prochlorococcus and no common trends emerged. Thus, the observations presented here does not support the 'translation-compensation' theory and instead suggest unique cellular elemental effects as a result of rising temperature among closely related phytoplankton lineages. Thus, the biodiversity context should be considered when predicting future elemental ratios and how cycles of carbon, nitrogen, and phosphorus may change in a future ocean
A Key Marine Diazotroph in a Changing Ocean: The Interacting Effects of Temperature, CO2 and Light on the Growth of Trichodesmium erythraeum IMS101
No full textTrichodesmium is a globally important marine diazotroph that accounts for approximately 60-80% of marine biological N2 fixation and as such plays a key role in marine N and C cycles. We undertook a comprehensive assessment of how the growth rate of Trichodesmium erythraeum IMS101 was directly affected by the combined interactions of temperature, pCO2 and light intensity. Our key findings were: low pCO2 affected the lower temperature tolerance limit (Tmin) but had no effect on the optimum temperature (Topt) at which growth was maximal or the maximum temperature tolerance limit (Tmax); low pCO2 had a greater effect on the thermal niche width than low-light; the effect of pCO2 on growth rate was more pronounced at suboptimal temperatures than at supraoptimal temperatures; temperature and light had a stronger effect on the photosynthetic efficiency (Fv/Fm) than did CO2; and at Topt, the maximum growth rate increased with increasing CO2, but the initial slope of the growth-irradiance curve was not affected by CO2. In the context of environmental change, our results suggest that the (i) nutrient replete growth rate of Trichodesmium IMS101 would have been severely limited by low pCO2 at the last glacial maximum (LGM), (ii) future increases in pCO2 will increase growth rates in areas where temperature ranges between Tmin to Topt, but will have negligible effect at temperatures between Topt and Tmax, (iii) areal increase of warm surface waters (> 18°C) has allowed the geographic range to increase significantly from the LGM to present and that the range will continue to expand to higher latitudes with continued warming, but (iv) continued global warming may exclude Trichodesmium spp. from some tropical regions by 2100 where temperature exceeds Topt
Towards understanding the prokaryotic contributions to cobalamin cycling in the Northwest Atlantic
No full textCobalamin has the potential to limit primary productivity and shape the structure and ecological interactions of marine microbial communities. The identification of major sources and sinks of this vitamin is needed in order to understand its availability in the ocean. In this thesis, assembly-based and short-read-based approaches were combined to analyze metagenomic samples from the Scotian Shelf and Slope region of the Northwest Atlantic. This resulted in the first identification of major producers, remodelers and consumers of cobalamin and related compounds in this region. Mass-spectrometry tools to monitor the contribution of Synechococcus, an important cyanobacterial group, to the cobalamin cycle in the Northwest Atlantic were also identified. The implementation of these tools in culture experiments enabled the identification of environmental and physiological factors with potential to affect cyanobacterial contributions to cobalamin cycling in this region. In sum, this thesis is a step towards elucidating the influence that cobalamin may have on marine primary productivity and microbial ecological interactions in the Northwest Atlantic
Distinctive Photosystem II Photoinactivation and Protein Dynamics in Marine Diatoms
No full textAbstract
Diatoms host chlorophyll a/c chloroplasts distinct from green chloroplasts. Diatoms now dominate the eukaryotic oceanic phytoplankton, in part through their exploitation of environments with variable light. We grew marine diatoms across a range of temperatures and then analyzed their PSII function and subunit turnover during an increase in light to mimic an upward mixing event. The small diatom Thalassiosira pseudonana initially responds to increased photoinactivation under blue or white light with rapid acceleration of the photosystem II (PSII) repair cycle. Increased red light provoked only modest PSII photoinactivation but triggered a rapid clearance of a subpool of PsbA. Furthermore, PsbD and PsbB content was greater than PsbA content, indicating a large pool of partly assembled PSII repair cycle intermediates lacking PsbA. The initial replacement rates for PsbD (D2) were, surprisingly, comparable to or higher than those for PsbA (D1), and even the supposedly stable PsbB (CP47) dropped rapidly upon the light shift, showing a novel aspect of rapid protein subunit turnover in the PSII repair cycle in small diatoms. Under sustained high light, T. pseudonana induces sustained nonphotochemical quenching, which correlates with stabilization of PSII function and the PsbA pool. The larger diatom Coscinodiscus radiatus showed generally similar responses but had a smaller allocation of PSII complexes relative to total protein content, with nearly equal stiochiometries of PsbA and PsbD subunits. Fast turnover of multiple PSII subunits, pools of PSII repair cycle intermediates, and photoprotective induction of nonphotochemical quenching are important interacting factors, particularly for small diatoms, to withstand and exploit high, fluctuating light.</jats:p
Determining diversity of freshwater fungi on decaying leaves : Comparison of traditional and molecular approaches
No full textLight challenge experiments conducted on <i>Ditylum brightwellii</i> P1S, P1B, P2S, and P2B (left to right) acclimated to 37 (top panels) and 287 µmol photons m
No full text<p><sup>−<b>2</b></sup><b> s</b><sup>−<b>1</b></sup><b> (bottom panels).</b> PSII repair capacity is estimated from the difference in <i>F</i><sub>V</sub>/<i>F</i><sub>M</sub> between the control (filled symbol) and lincomycin (open symbol) treatments. The susceptibility to photoinactivation is estimated from the change in <i>F</i><sub>V</sub>/<i>F</i><sub>M</sub> in the lincomycin treatment. Vertical dashed lines indicate the start (<i>t</i> = 0) and end (<i>t</i> = 90) of the high light challenge. <i>F</i><sub>V</sub>/<i>F</i><sub>M</sub> measurements are relative to <i>F</i><sub>V</sub>/<i>F</i><sub>M</sub> at t = 0.</p
The fraction of PSII centres that are open (qp) is plotted at 30 μmol photons m<sup>-2</sup> s<sup>-1</sup> (red) and then in the first step of the treatment light (260 μmol photons m<sup>-2</sup> s<sup>-1</sup>) (blue).
No full textThe fraction of PSII centres that are open (qp) is plotted at 30 μmol photons m-2 s-1 (red) and then in the first step of the treatment light (260 μmol photons m-2 s-1) (blue).</p
A Hard Day's Night: Diatoms Continue Recycling Photosystem II in the Dark
No full textMarine diatoms are photosynthetic, and thrive in environments where light fluctuates. Like all oxygenic photosynthetic organisms diatoms face a light-dependent inactivation of the Photosystem II complexes that photooxidize water to generate biosynthetic reductant. To maintain photosynthesis this photoinactivation must be countered by slow and metabolically expensive protein turnover, which is light dependent in cyanobacteria and in plants. We tracked daily cycles of the content, synthesis and degradation of Photosystem II, in a small and in a large marine diatom, under low and high growth light levels. We show that, unlike plants, diatoms maintain extensive cycling of Photosystem II proteins even in the dark. Photosystem II protein cycling saturates at low light, and continued cycling in dark periods, using energy from respiration, allows the diatoms to catch up to excess photoinactivation accumulated over the preceding illuminated period. The large diatom suffers only limited photoinactivation of Photosystem II, but cycling of Photosystem II protein exceeds Photosystem II inactivation, so the large diatom recycles functional Photosystem II units before they are inactivated. Through the diel cycle the contents of active Photosystem II centers and Photosystem II proteins change predictably, but are not correlated, generating large changes in the fraction of total PSII that is active at a given time or growth condition. We propose that dark and steady cycling of Photosystem II proteins is driven by the tight integration of chloroplastic and mitochondrial metabolism in diatoms. This ability for baseline, continuous Photosystem II repair could contribute to the success of diatoms in mixed water environments that carry them from illumination to darkness and back
Photosystem II maximum quantum yield (F<sub>v</sub>/F<sub>m</sub>) (A, B) or Photosystem II quantum yield for electron transport (Φ<sub>PSII</sub>) (C, D, E, F) in <i>Synechococcus</i> (A, C, E) or <i>Synechocystis</i> (B, D, F) over a 120 hour iron depletion timecourse.
No full text<p>(A, B) F<sub>v</sub>/F<sub>m</sub> measured from cells under 0 μmol photons·m<sup>−2</sup>·s<sup>−1</sup>. (C, D) Φ<sub>PSII</sub> measured from cells under the growth light level of 65 μmol photons·m<sup>−2</sup>·s<sup>−1</sup>. (E, F) Φ<sub>PSII</sub> measured from cells under saturating light of 262 μmol photons·m<sup>−2</sup>·s<sup>−1</sup>, 4X higher than the growth light level. Data were compiled from 6 (<i>Synechococcus</i>) or 5 (<i>Synechocystis</i>) replicate measurements from 6 or 5 separate cyanobacterial cultures. All yield data were captured using blue light excitation of fluorescence. Data presented are mean +/− standard error, n = 5 or 6.</p
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